1. Field of the Invention
This invention relates to a cassette for housing therein a sheet-shaped radiation image recording medium, such as X-ray photographic film or a stimulable phosphor sheet, such that the sheet-shaped radiation image recording medium is capable of being taken out therefrom.
2. Description of the Prior Art
X-ray photographic film has heretofore been known as a recording medium, on which a radiation image of an object, such as a human body, is recorded. Also, as a recording medium used for the same purposes, a sheet provided with a layer of a stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) has heretofore been known. Such stimulable phosphor sheets are disclosed in, for example, U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395. When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
A radiation image of an object is recorded on a stimulable phosphor sheet by, for example, exposing the stimulable phosphor sheet to radiation, which has passed through the object. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned two-dimensionally with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected. In this manner, an electric image signal representing the radiation image can be obtained.
By way of example, the stimulable phosphor sheet or the X-ray photographic film described above is housed in a light-tight cassette. Each cassette serves to house a single sheet-shaped image recording medium, such as a stimulable phosphor sheet, therein. In general, the cassette is constituted of a box member provided with an opening, through which a recording medium is to be fed into and out of the box member, at a portion, and a cover member, which opens and closes the opening.
When the cassette is used during an operation for recording a radiation image on the stimulable phosphor sheet, or the like, a sheet-shaped image recording medium is housed in the cassette, and the cassette is located at a position for exposure to, for example, radiation having passed through an object. The sheet-shaped image recording medium, which has been housed in the cassette, is exposed to radiation, and a radiation image is thereby recorded on the sheet-shaped image recording medium. After the radiation image has been recorded on the sheet-shaped image recording medium, the sheet-shaped image recording medium is taken out of the cassette. In cases where the sheet-shaped image recording medium is a stimulable phosphor sheet, the radiation image stored on the stimulable phosphor sheet is read out by exposing the stimulable phosphor sheet to stimulating rays. In cases where the sheet-shaped image recording medium is X-ray photographic film, the radiation image recorded thereon is developed.
As a cassette for housing a sheet-shaped radiation image recording medium therein, a cassette is proposed in, for example, U.S. Pat. No. 4,987,308, wherein a cushioning material constituted of a foamed material is adhered to an inner surface of a flat plate-like portion of the cassette, which portion stands facing an image recording layer of the sheet-shaped radiation image recording medium. (The image recording layer is a stimulable phosphor layer in cases where the sheet-shaped radiation image recording medium is a stimulable phosphor sheet, or is a photosensitive material layer in cases where the sheet-shaped radiation image recording medium is X-ray photographic film.) The foamed material is provided in order to prevent the image recording layer from rubbing against the inner surface of the cassette and from being thereby scratched when the sheet-shaped radiation image recording medium is fed into and out of the cassette.
However, the conventional cassette utilizing a foamed material as the cushioning material has the drawbacks in that the foamed material crumbles and constitutes dust while the cassette is being used repeatedly and in that, if dust enters the cassette somehow, dust will intervene between the foamed material and the image recording layer of the sheet-shaped image recording medium and will scratch the image recording layer. Also, the coefficient of friction of the foamed material varies due to fluctuations in the degree of foaming between production lots or due to whether the foamed material is new or old (i.e. due to the use period). If the coefficient of friction is large, the problem will occur in that, when the sheet-shaped image recording medium slips on the foamed material and is thus fed into and out of the cassette, the resistance to slip on the foamed material becomes large and, therefore, the sheet-shaped image recording medium cannot be smoothly fed into and out of the cassette. The conventional cassette also has the drawbacks in that, even if the frictional resistance does not fluctuate, foamed materials have a comparatively large coefficient of friction and do not always allow the sheet-shaped image recording medium to be smoothly fed into and out of the cassette.